1. Field of the Invention
The present invention relates to a method of forming a metal line of a semiconductor memory device, and more specifically, to a method of forming a metal line of a semiconductor memory device, wherein generation of malfunction, which is caused due to a bridge generating between a metal line electrically connected to a drain contact plug, and a source contact plug, can be prevented.
2. Discussion of Related Art
Generally, in semiconductor memory devices, as wiring technology for forming a metal line, technologies in which a conductive film is deposited on an insulating film and is then patterned to form a metal line by means of a photolithography process and an etch process has been widely used.
This metal line serves to transfer an externally applied driving voltage (bias voltage) to an underlying semiconductor structure layer. In order to electrically connect the metal line and a predetermined semiconductor structure layer, a contact plug is needed.
In NAND flash memory devices of the semiconductor memory devices, a source contact plug (SRCT) and a drain contact plug (DRCT) are formed as contact plugs. The source contact plug serves to connect a source region and a predetermined metal line, and the drain contact plug electrically connects a drain region and a predetermined metal line.
Hereinafter, a conventional method of forming a metal line of a NAND flash memory device will be described with reference to FIGS. 1a to 1h, and FIGS. 2a to 2f. In this case, FIGS. 1a to 1h are cross-sectional views taken in a Y axis (a bit line direction), and FIGS. 2e to 2f are cross-sectional views taken in an X axis (a word line direction).
Referring to FIG. 1a, a semiconductor substrate 10 in which a gate electrode 16, and a source and drain region (not shown) are formed is provided. In this case, it is assumed that the gate electrode 16 includes a tunnel oxide film 11, a floating gate 12, a dielectric film 13, a control gate 14 and a conductive layer 15 for convenience of explanation.
Spacers 17 are formed on both sidewalls of the gate electrode 16. A nitride film 18 and an interlayer insulating film 19 (hereinafter, referred to as “first interlayer insulating film”) are then formed on the entire structure including the spacers 17 in a sequential manner. The first interlayer insulating film 19 is then polished.
Referring to FIG. 1b, a source contact mask (not shown) is formed. The first interlayer insulating film 19 is etched by means of an etch process using a source contact mask by using the nitride film 18 as an etch-stop layer. A strip process and a cleaning process are then performed to remove the nitride film 18, which is exposed through the patterned first interlayer insulating film 19, while removing the source contact mask, thereby forming a source contact hole 20 through which the source region is exposed.
Referring to FIG. 1c, a source contact plug 21 connected to the source region is formed so that the source contact hole 20 is gap-filled. An interlayer insulating film 22 (hereinafter, referred to as “second interlayer insulating film”) is then formed on the entire structure including the source contact plug 21.
Referring to FIGS. 1d and 2a, a drain contact mask (not shown) is formed on the second interlayer insulating film 22. An etch process using the drain contact mask is implemented to remove the second interlayer insulating film 22 and the first interlayer insulating film 19, thus forming a drain contact hole (not shown) through which the drain region is exposed. Thereafter, after a conductive layer for a drain contact plug is deposited so that the drain contact hole is gap-filled, a polishing process is performed to form a drain contact plug 23.
In this case, the second interlayer insulating film 22 is recessed in a predetermined thickness by means of the polishing process, and a thickness of the second interlayer insulating film 22 becomes thin accordingly.
Referring to FIGS. 1e and 2b, a nitride film 24 is formed on the entire structure including the drain contact plug 23.
Referring to FIGS. 1f and 2c, an insulating film 25 (hereinafter, referred to as “third interlayer insulating film”) is deposited on the nitride film 24.
Referring to FIGS. 1g and 2d, after a trench mask 26 is formed, an etch process using the trench mask 26 is performed to pattern the third interlayer insulating film 25.
Referring to FIG. 2e, a cleaning process is implemented to remove the nitride film 24, which was used as the etch-stop layer in the etch process. Thereby, a trench 27 through the drain contact plug 23 is exposed is formed.
Referring to FIGS. 1h and 2f, after a conductive layer is deposited on the entire structure so that the trench 27 is gap-filled, a polishing process is performed to form a metal line 28 that is electrically connected to the drain contact plug 23.
In the conventional method of forming the metal line of the NAND flash memory device, which has been described with reference to FIGS. 1a to 1h and FIGS. 2a to 2f, the thickness of the second interlayer insulating film 22 that provides electrical insulation between the metal line 28 and the source contact plug 21 is reduced twice. As shown in FIGS. 1d and 2a, in the polishing process for forming the drain contact plug 23, the thickness is firstly reduced. Next, the thickness is secondly reduced by means of over-etch (see ‘A’ in FIG. 2e) that is performed so as to remove residue, which is generated in the removal process of the nitride film 24 for forming the trench 27 in FIGS. 1g and 2e. 
As such, if the thickness of the second interlayer insulating film 22 reduces, a bridge is formed between the metal line 28 and the source contact plug 21. This is because the thickness of the second interlayer insulating film 22, which provides electrical insulation between the metal line 28 and the source contact plug 21, becomes thin. The drain contact plug 23 and the source contact plug 21 are typically formed parallel to each other, and the metal line 28 intersects the top of the source contact plug 21 at an angle of 90° with the second interlayer insulating film 22 therebetween. This is because if the thickness of the second interlayer insulating film 22 reduces, a bridge is generated between the metal line 28 and the source contact plug 21. This makes fragile insulation margin between the source contact plug 21 and the metal line 28. Thus, if a bridge is generated between the source contact plug 21 and the metal line 28, a device operate erroneously.